Pseudotachylyte in MeteoritesShow simple item record
| dc.creator | University of Texas publication | |
| dc.creator | Barnes, Virgil E. (Virgil Everett), 1903-1998 | |
| dc.date.accessioned | 2024-02-12T18:10:02Z | |
| dc.date.available | 2024-02-12T18:10:02Z | |
| dc.date.issued | n.d. | |
| dc.identifier.uri | https://repository.tcu.edu/handle/116099117/63076 | |
| dc.description | An article by Virgil E. Barnes that includes discussion of the Bluff meteorite and figures. From pages 646 to 657. | |
| dc.relation | Oscar Monnig Papers (MS 124) | |
| dc.rights | Prior written permission from TCU Special Collections required to use any document or photograph. | |
| dc.source | Series III, Box 06, Fayette Co., Texas literature folder | |
| dc.subject | Meteorite | |
| dc.subject | Fayette County meteorite | |
| dc.subject | Fayette County (Tex.) | |
| dc.subject | Bluff meteorites | |
| dc.subject | University of Texas | |
| dc.subject | Pseudotachylyte | |
| dc.subject | Metallography | |
| dc.subject | Smithsonian Institution | |
| dc.subject | Barnes, Virgil E. | |
| dc.title | Pseudotachylyte in Meteorites | |
| dc.type | Article | |
| dc.description.transcription | PSEUDOTACHYLYTE IN METEORITES Virgil E. Barnes In a paper by Hall and Molengraaff describing the geology of the Vredefort Mountain Land, an extensive discussion of pseudotachylyte is given. The similarity of this material to veins found in the Kimble County" and Bluff meteorites is striking. With this in mind the writer restudied and made other thin sections of the Kimble County meteorite and borrowed the U.S. National Museum's thin sections of the Bluff veins described by Merrill. For con-parison the writer also prepared thin sections of pseudotachylyte and myolinite found recently in the pre-Cambrian area of central Texas. Shand was the first to use the word pseudotachylyte. He states “… the name pseudotachylyte has been adopted in recognition of the fact that these rocks have a great similarity to tachylyte, also that such rocks have been mistaken for trap and tachylyte in Scotland and India as well as in South Africa, and for the further reason that no more suitable name is in existence." The pseudotachlyte described by Shand is in granite. It is dense, black, and contains inclusions of quartz and feldspar derived from the granite. Shand recognized microscopically three types of pseudotachylyte ranging from a type that is dense, clouded with magnetite, and slightly isotropie; through one that shows an incipient crystallization of small grains and scales which are pleochroic from yellowish green to pale grass-green; to a type consisting either of a honeycomb of polygonal spherulites of a dark brown color, or of a felt of feldspar-microlites with a subordinate amount of magnetite dust and a few green scales in the interstices. 646 The University of Texas Publication No. 3945 Shand places pseudotachylyte in the following continuous series of crushed to recomposed rocks: myolinite, fritted myolinite or flinty crushrock, fused myolinite or pseudotachylyte, and recrystallized pseudotachylyte. In an appendix to his paper he states: "The pseudotachylyte has originated from the granite itself through melting, caused not by shearing but by shock, or alternatively by gas-fluxing. Hall and Molengraaff in a more extended examination of this same area (Paris and the Vredefort Mountain Land of South Africa) found pseudotachylyte in all of the old rocks of the area including the metamorphosed sediments. They note also that magnetite separates out in the wall rocks and that crushing next to the veins is prevalent with few exceptions. They concluded that the pseudotachylyte was produced by crushing without much shearing and that shock and gas-fluxing were probably not operative. Shand, as well as Hall and Molengraaff, shows by chemical analyses that the pseudotachylyte is very similar in composition to that of the enclosing rocks and concludes that it is derived from these rocks and that it has not migrated far. Crickmay in a study of myolinites in Georgia notes that: The myolinite is similar to pseudotachylite (Hall and Molengraaff, 1925) in being pseudo-eruptive, but there has been no fusion in its development. If the presence of glass is taken as distinctive of a pseudotachylite, the name is not applicable here. However, if the term is used to denote a myolinite with pseudo-eruptive relations to the parent rock, it is certainly descriptive of the myolinite near Neel's Gap. Waters and Campbells have made an excellent review of myolinites and, more to the point, have restudied a few samples of the South African pseudotachylyte using a different method of attack. They failed to find true glass either in this or similar material from California. The index of refraction of the isotropic-appearing pseudotachylyte is considerably higher than that of the same material when fused into a true glass by the oxyacetylene torch. Furthermore, under high magnification, 250x, they find that much of the pseudotachylyte that appears isotropic at low magnification is resolvable into definite particles. The inferred conclusion, therefore, is that pseudotachylyte is in reality a very finely divided myolnile. Black veinlets are very common in meteorites, and probably more than half of all stony meteorites are veined. The only veins examined for this discussion, however, are those found in the Kimble County, Bluff, and Cuero meteorites. The latter is not truly veined but is faulted with myolinite along the faults. The veinlets in the Kimble County meteorite are narrow, being less than a millimeter thick. Figure 99 a tracing showing the veinlets present on a Fig. 99. Tracing of veins on a polished surface of the Kimble County meteorite. polished surface of the Kimble County meteorite. Two prominent directions intersecting at about 70° are present. On another polished section one of these veins cuts across a light-colored chondrule, and so far as can be seen there has been no offsetting of the chondrule. By reflected light the veins of both the Kimble County and Bluff meteorites have a bronzy appearance and have some metal distributed throughout. In the Bluff meteorite studied by Whitfield and Merrill, a vein about 2 millimeters thick was found. A chemical analysis was made of the vein which compares rather closely to that of the meteorite as a whole. Individual stones of Pultusk and Mocs seem to be made up wholly of vein material.' Analyses of veins more than an inch in thickness in the Orvinio and Ställdalen meteorites have also been made, and in each case the veins are similar in composition to that of the meteorites as a whole. These analyses are given in the following table: In thin sections the veinlets are entirely opaque except for color. less inclusions which remain bright during rotation between crossed nicols. The edges of some of the larger olivine crystals where they contact the veins have the same appearance. The inclusions are, therefore, mostly altered olivine, with possibly some altered hypersthene being present. In both the Kimble County and Bluff meteorites the olivine and hypersthene are clouded by minute black specks. In the olivine of the Kimble County meteorite, especially these black specks are arranged mostly in parallel zones without respect to the orientation of the olivine crystal but with a more or less parallel orientation throughout the thin section. This is suggestive of incipient shearing with magnetite separating from the femie minerals along the shearing direction. This may be similar to the separation of magnetite reported by Hall and Molengraaffs in the wall rocks adjacent to pseudotachylyte. Some of the colorless inclusions in the meteorite veinlets are practically free of these black specks. From the rounded and embayed outline of these inclusions and their changed optical character, they must have been near their melting point and possibly plastic enough for the magnetite to migrate into the vein proper. In Plate 31, microphotographs of these veins are shown. Figure 1 of this plate is a photograph by plane-polarized light of a vein in the Bluff meteorite taken from one of Merrill's thin sections. Figure 2 is the same except that crossed nicols were used. Figures 3 and 4 are photographs of a vein in the Kimble County meteorite using plane-polarized light and crossed nicols respectively. Figures 5 and 6, using plane-polarized light and crossed nicols respectively, are photographs of pseudotachylyte from an outrop in the bed of Sandy Creek near the base of Enchanted Rock in Llano County, Texas. It was hoped that the difference in character of the altered and normal olivine could be clearly shown by photographs, but unfortunately this is not possible at present. In Plate 31, figure 2, many of the inclusions in the vein remain bright during 360° of rotation, and in Plate 31, figure 4, the inclusion in the thickened portion of the vein at the right does the same. The photographs do not show this characteristic well but do show something of the character of the narrow veins. A portion of the vein wall situated at the lower side of the vein in Plate 31, figures 3 and 4, is sharply defined even though the wall rock is of heterogeneous material. This is indicative of a clean-cut shear, but, as already mentioned, a chondrule is crossed by one of these veins, and no displacement can be seen. The amount of movement to produce this shearing must be minute; therefore, the containing pressure must have been very high for friction to produce enough heat to change the optical properties of the inclusions. In Plate 31, figures 1 and 2, a veinlet is shown which extends almost through the pseudotachylyte and into the wall rock on one side. Quartz veinlets having a similar appearance have been pictured by Waters and Campbell." The pseudotachylyte from Llano County shows many of the characteristies of the normal non-recrystallized pseudotachylyte described from South Africa. In thin sections and in the accompanying photographs of a thin section (PI. 31, figs. 5 and 6), the black, dense material is pseudotachylyte which has islands of microcline and quartz distributed throughout. The extreme crushing and shearing and "milling out" of the minerals are shown clearly in this section. The edge of the large feldspar crystal is frayed and drawn out. Using crossed nicols, the de. formation of the crystal is shown be the bent lamellae and the undulatory extinction. In this section there is some material which remains bright during 360° of rotation between crossed nicols, thus indicating a common genetic relationship between the pseudotachylytes of meteorites and of earthly rocks. The veins in the meteorites differ considerably from those in the granite. However, other than the fact that the granite contains a larger number of fragments, the difference is chiefly due to the great dissimilarity in mineralogical and chemical composition of the two. Hall and Molengraaff, is in describing pseudotachylytes from much less basic rocks, state: "The groundmass or matrix of the pseudotachylyte is generally crowded with small black specks, taken to be magnetite, which may be so abundant that the rock becomes only very little transparent and not resolvable under the microscope even with high powers." It is believed that the greater opacity of the veins found in the Kimble County and Bluff meteorites may be accounted for by the high iron content of the meteorite minerals, which has separated out as magnetite. In consideration of the small number of pseudotachilvtes described from North America, the field relations of this rock will be given. This pseudotachylyte is from a few hundred feet within the Enchanted Rock granite mass and was collected from a shear zone of dull brick-red, fritted myolinite or flinty crush rock 20 feet thick which strikes N. 33° E. and has a vertical dip. The pseudotachylyte, which constitutes less than 1 per cent of the shear zone, is present as dense, Minty-appearing, black, narrow, resistant streaks. Pseudotachylyte from the type locality is not available for comparison, but nevertheless the Llano material is easily identified when compared with the microphotographs of pseudotachylyte in Hall and Molengraaff's memoir. In the Llano pseudotachylyte, and as pointed out by Crickmay for Georgia and by Waters and Campbell for California and South Africa, no material is present that can be definitely identified as glass. Much entirely opaque material is present in which glass might be contained but in which it would be difficult to prove the presence of glass since light cannot be transmitted through it. The extremely small translucent particles all appear to be anisotropic, even though some of them are weakly so and do not show definite extinction. It is possible that these particles are altered in the same manner as the much larger olivine fragments observed in meteorite veins. It is thought that enough heat has been created to disorder somewhat the molecular structure but not to destroy it entirely. This may be the characteristic noted by Hall and Molengraaff's and called "incipient melting." Minerals during such disordering might be plastic enough to migrate and at the same time not be a glass. Likewise it seems that reerystallization of this material could take place if the temperature remained high enough for a sufficient length of time, but if the original thesis that heat is developed by crushing and shearing is correct, then it would be expected that the heat would dissipate rapidly into the uncrushed rock before recrystallization could take place. The word pseudotachylyte should probably be reserved for a myolinite in which the character of the minerals has been changed through crushing and heating, with a separating of dark material and the development of very small particles, many of which are molecularly disordered. These latter are then neither glass nor are they definitely mineral, and, therefore, pseudotachylyte, or "false tachylyte," seems entirely appropriate. "Pseudotachylyte vein" should be retained for that pseudotachylyte which shows definitely an intrusive relationship. In discussing meteorites, however, it is difficult to think of the pseudotachylyte as being other than veins since this type of material has always been referred to as veins. If a certain amount of interchangeable usage is noticed in this paper, let it be understood that veins, unless otherwise qualified, mean pseudotachylyte. Veins have been described in the Cuero's meteorite which prob ably corresponds more closely to a myolinite. In this case the veins are along slickensided faults of small displacement. The Cuero is an old fall on which the fluted surface is preserved, yet open cracks extend to a depth of at least 6 centimeters along the faults, indicating that the filling is a myolinite which, because of its granularity, weathered easily and was removed If all veined meteorites were examined, undoubtedly all gradations from myolinite barely aggregated sufficiently to hold the meteorite together, to the densest of pseudotachylyte would be found. That these veins were formed after the meteorite entered the atmosphere or that they formed upon hitting the ground can probably safely be disregarded. It is true that these veins resemble somewhat the crust of a meteorite. but upon one moment's reflection it is seen that, even though the surface of the meteorite may become incandescent, it is in the atmosphere only for a few seconds; and that due to slow conduction the interior during this time must remain at a temperature near that of interplanetary space. Under these conditions a surface melt would immediately congeal and would not be able to penetrate cracks deeper than a few centimeters at most. Another possible origin of these veins that should be mentioned is an origin by shock such as would be caused by the disaggregation of a celestial body during a close approach or collision. With this origin, however, it is difficult to understand how myolinite along faults such as is found in the Cuero could form and the meteorite not be parted along these weak zones. Recognition of the fact that vein material in meteorites is similar to material found on the earth leads to the substantiation of the postulate that meteorites are derived by the disaggregation of celestial bodies. Some idea of the size of such a body might be obtained if the magnitude of the forces necessary to produce pseudotachylyte were known. This material has been found on the earth only in metamorphosed rocks and must have formed at depths of several miles below the surface and consequently under rather high pressures. In a celestial body a certain minimum size would be necessary before deformative movements could take place. Furthermore. if the body were zoned with nickel iron in the center, followed in succession by zones of pallasitic material and stone containing some iron, then the pseudotachylyte would have been in the outer stony part and not near the center of the body. This gives an additional size below which the body could not have been and still have the pseudotachylyte formed within it. This indicates that the body would have to be at least of a minimum size of several hundred miles in diameter. Some indication of the upper limit of size possible for such a body can he obtained if it is assumed that the meteorites seen to fall on earth are samples of a body from which they were originally derived and that pseudotachylyte has a maximum pressure limit above which it cannot form. According to the available sample using the 60 falls observed in North America up to January 1, 1909, as given in the following table, an indication the proportion of veined meteorites can he obtained Veined meteorites make up nearly half of all the meteorites observed to fall in North America. The figures for the entire earth will differ somewhat from these but in general will be comparable. A factor that may be significant and may tend to nullify the above averages is the possibility that meteorites are derived from many different bodies. Also many of the more friable chondrites may not have reached the ground, thus giving a too high proportion for the veined stones. However viewed, if the present sample is at all reliable, a considerable portion of the meteorite source body (or bodies) is composed of veined rocks. If there are limits to the conditions under which these veins form, then the size of the body would also have to be limited probably at the most to a few thousand miles in chameter The identification of pseudotachylyte in meteorites confirms Hall and Molengraaff's belief that gas fluxing is not operative in the formation of such material found on earth. Meteorites are poor in gases and in minerals suggestive of a gaseous or aqueous origin. It becomes very unlikely, therefore, that gases could be operative in forming these veins in the bodies from which meteorites are derived. Crushing and shearing under a considerable load seem to be the logical source for the heat which modified the minerals into the substances found in these veins. The intrusive nature of this material is undoubtedly due to differential movement set up during crushing. A rock mass cannot be deformed without developing tensional strain in some direction. If this strain is sufficient to cause fractures and if these fractures penetrate to a crush zone in which the products of crushing are under high containing pres. sures, then the crushed material will flow toward the point of lowest pressure, providing the differential pressure is sufficient to overcome the frictional resistance. In this manner the presence of intrusive crush rocks can be explained. The main attempt in this paper is to show that the black veins in meteorites are very similar to pseudotachylyte found in some of the metamorphosed rocks of the earth and that their origin is probably similar. This, in combination with the proportion veined meteorites observed to fall over a given length of time, leads to the tentative conclusion that the source of meteorites must be from a body (or bodies) of the order of magnitude of several hundred to a few thousand miles in diameter. PLATE 31 Microphotographs of pseudolachsiste in meteorites and in cranite. 1. Pseudotachylyte vein (black) in the Bluff meteorite. Photographed with plane-polarized light. The inclusions in the vein are mostly mitered ovine small veinlet which cuts the pseudotachylyle and the inclusions penetrates the wall rock. About ×24. 2. Pseudotachsiste vein (black in the Bluff meteorite (same view as fig. 1. Photographed with crossed nicols. A few of the inclusions are so altered that they remain bright during 360° of rotation between crossed nicols. About ×20. 3. A complex vein of pseudotachylyte in the Kimble County meteorite which shows intrusion of pseudotachviste into cross fractures at the left. Photographed with plane-polarized light. About x24. 4. A complex vein of pseudotachsiste in the Kimble County meteorite (same view as fig. 3. Photographed with crossed nicols. The inclusion in the thickened portion of the vein, at the right, remains bright during 360 of rotation between crossed nicols. About x20. 5. Pseudotachylyte from the Enchanted Rock granite mass, Llano County. Texas. Photographed with plane-polarized light. The light-colored bands in the upper part of the photograph are composed largely of finely divided fragments of quartz and feldspar. The black material is pseudotachriste which is so opaque that light cannot penetrate it in sections 003 mm. in thickness, About x38. 6. Pseudotachylyte from the Enchanted Rock granite mass, Llano County, Texas (same view as fig. 6). Photographed with crossed nicols, This photograph brings out more clearly the fine state of division of the fragmented quartz and feldspar. The large feldspar crystal at the bottom is highly strained as is shown by the undulatory extinction and the bent twinning lamellae. About x32. |
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